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Figure 6. Northern fur seal pup production on St. Paul and St. George Islands. The average number of pups born over the past three censuses is multiplied by a correction factor of 4.47 to estimate the total stock for the Pribilof Islands.

The Pribilof Islands—St. Paul and St. George, Alaska, (Fig. 5)—are home to the largest breeding colony of northern fur seals (Callorhinus ursinus) in the United States. This population has shown an overall pattern of decline since the mid-1950s (Fig. 6), although rates vary between St. Paul and St. George Islands. During a brief period of stability (1995-96), a previous study found that fur seals segregated foraging habitat between and within islands (Robson et al. 2004, Canadian Journal of Zoology). This foraging habitat segregation may have been a mechanism to reduce competition among fur seals at these large breeding colonies (>950,000 fur seals in 1996).

By 2010, the population of northern fur seals on the Pribilof Islands had declined to ~560,000 fur seals. If the segregation of foraging areas was due to intraspecific competition, we would expect that after this significant population decline, habitat segregation may be relaxed and foraging effort reduced as fewer fur seals compete within the foraging areas.

To re-examine habitat segregation within this population, scientists from the Alaska Ecosystems Program equipped 27 adult female northern fur seals with GPS tracking instruments to measure at-sea behavior between August and October 2010. Instrument deployments were distributed between two rookeries on St. George Island and between two natal areas, "northeast" (Polovina Cliffs and Vostochni rookeries) and "southwest" (Reef and Zapadni Reef rookeries), on St. Paul Island.

We calculated trip durations, maximum distance travelled from the rookery (during each foraging trip), and foraging habitat (95% fixed kernel home range) for all fur seals on St. George Island and the two natal areas on St. Paul Island to make comparisons between the two studies (1995-96 and 2010).

The general patterns of island-wide habitat use and segregation were similar between studies (Fig. 5). In 2010, foraging habitat segregation was found between islands, as less than 8% of foraging trips by St. Paul Island fur seals occurred in the St. George fur seal foraging area. This is comparable to the 1995-96 study, where 12% of the foraging trips by St. Paul Island fur seals occurred in the St. George fur seal foraging habitat.

Among the three foraging regions (northeast, southwest, and St. George), overlap of total foraging areas in 2010 ranged from 4.0% to 27.8%, which was slightly higher than the 7.3%-16.5% measured in 1995-96. On St. George Island, no reduction in foraging effort was found as fur seals travelled further in 2010, and trip durations did not change between study years (Table 3). On St. Paul Island, there was a trend for shorter foraging trips in 2010 (Table 3). And, although the maximum distance travelled during all foraging trips was similar for fur seals on St. Paul Island in both study years, the foraging ranges for fur seals from each natal area were 69%-76% larger in 2010 (Table 3).

Our results suggest that fur seals are not experiencing the level of reduced competition that may be expected from a population that has been nearly halved in size (41% population decline). This indicates that fur seals are expending similar effort to acquire sufficient prey and suggests a change in fur seal carrying capacity may have occurred in the Bering Sea. Further investigation is necessary to understand the role of inter-annual variability in fur seal foraging behavior and to assess the mechanism(s) of a possible change in carrying capacity.

Changes to carrying capacity can result from a reduction in prey availability (e.g., Boyd et al. 1994, Journal of Animal Ecology; Costa 2008, Aquatic Conservation: Marine and Freshwater Ecosystems) or increased competition with other predators, including fisheries (e.g., Barlow et al. 2002, Marine Biology; Ainley et al. 2006, Ecology). Due to the continued foraging habitat segregation within and between islands, natural and anthropogenic disturbances may have differential impacts on fur seals based on their breeding location. Consequently, a complex management or conservation strategy may be needed to help this population recover from its current downward trajectory.